How to find low side and high side on an a/c unit

John’s question:

Which is the low side and high side on an a/c unit”


The low side is generally the thicker AC line while the high side is the thinner diameter line. On all modern cars with 134 A refrigerant, the low side line has a smaller diameter port fitting where you add the refrigerant once you screw the cap off. The high side add port has a larger diameter port fitting where you check the high pressure with a gauge set. If you get a fitting to add refrigerant to the system, you will find the fitting for adding refrigerant will ONLY fit the low side. It will be too small to fit on the high side line on any 134 A refrigerant system. And if you have a set of AC gauges, you’ll notice the low side should run between thirty and forty pounds of pressure with the compressor running, and the high side would be 200 to 300 pounds.

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